The present invention generally relates to methods of preparing microparticles, and more particularly to methods of preparing microparticles encapsulating biologically active agents, such as therapeutic agents.
Several methods for preparing microparticles are well known in the art. Examples of these processes include single and double emulsion solvent evaporation, spray drying, solvent extraction, solvent evaporation, phase separation, simple and complex coacervation, and interfacial polymerization. Methods developed for making microparticles for drug delivery are described in the literature, for example, in Doubrow, ed., "Microcapsules and Nanoparticles in Medicine and Pharmacy" (CRC Press, Boca Raton 1992) and Benita, ed., "Microencapsulation: Methods and Industrial Applications" (Marcel Dekker, Inc., New York 1996).
Emulsion-based processes usually begin with the preparation of two separate phases: a first phase, which generally consists of a dispersion or solution of an active agent in a solution of polymer dissolved in a first solvent, and a second phase, which generally consists of a solution of surfactant and a second solvent that is at least partially immiscible with the dispersed phase. After the first and second phases are prepared, they are combined using dynamic or static mixing to form an emulsion, in which microdroplets of the first phase are dispersed in the second, or continuous, phase. The microdroplets then are hardened to form polymeric microparticles that contain the active agent. The hardening step is carried out by removal of the first solvent from the microdroplets, generally by either an extraction or evaporation process.
Several U.S. patents describe solvent removal by extraction. For example, U.S. Pat. No. 5,643,605 to Cleland et al. discloses an encapsulation process in which the emulsion is transferred to a hardening bath (i.e. extraction medium) and gently mixed for about 1 to 24 hours to extract the polymer solvent. The long period of time required for extraction is undesirable, particularly if the process is to be operated continuously. Others have disclosed processes that compensate for the unfavorable thermodynamics (slow and incomplete extraction) by using a large excess of extraction medium. For example, U.S. Pat. No. 5,407,609 to Tice et al. teaches transferring the emulsion to a volume of extraction medium that is preferably ten or more times the volume required to dissolve all of the solvent in the microdroplets, so that greater than 20-30% of the solvent is immediately removed. U.S. Pat. No. 5,654,008 to Herbert et al. similarly discloses a process in which the volume of quench liquid, or extraction medium, should be on the order often time the saturated volume. The use of a large excess of extraction medium rapidly extracts a portion of the solvent from each microdroplet, creating a concentration gradient within each droplet and forming a polymer skin on the surface that advantageously traps active agent, but which disadvantageously slows extraction of the remaining solvent from the center portion of the microdroplet. Larger volumes of extraction medium also may increase process equipment and operating costs, as well as the costs associated with recycling or disposing of used extraction medium.
Evaporation is another approach known in the art for solvent removal. For example, U.S. Pat. No. 3,891,570 to Fukushima et al. and U.S. Pat. No. 4,384,975 to Fong teach solvent removal by evaporating an organic solvent from an emulsion, preferably under reduced pressure or vacuum. Solvent evaporation processes generally occur slowly enough such that the solvent/polymer composition remains uniform throughout each microdroplet during the evaporation step, such that a polymer skin is not created. For this same reason, however, the evaporation process is not favored for use with many active agents that partition into the continuous phase, resulting in significant loss of active agent into the continuous phase and/or the extraction medium, and consequently poor loading of active agent in the microparticle. Evaporation, however, would be highly desirable if such partitioning could be substantially avoided, since no extraction phase solvent and associated tanking and piping are require as in the extraction process.
One effort combining evaporation and extraction is disclosed in U.S. Pat. No. 4,389,330 to Tice et al. ("Tice '330"). Tice '330 describes an emulsion-based method for making drug-loaded polymeric microspheres that uses a two-step solvent removal process: evaporation followed by extraction. The evaporation step is disclosed to be conducted by application of heat reduced pressure, or a combination of both, and is purported to remove between 10 and 90% of the solvent. Tice '330 also discloses that the extraction medium with dissolved solvent must be removed and replaced with fresh extraction medium, preferably on a continual basis. Consequently, the process requires either large volumes of extraction medium or an intermediate isolation of the microspheres combined with a change in the composition of the extraction medium.
Li et al., J. Controlled Release 37:188-214 (1995) describes a model of a solvent removal process in which an emulsion of the dispersed phase is formed in a continuous phase devoid of dispersed phase solvent, extracting a portion of the dispersed phase solvent into the original volume for a brief period of time, and then further extracting the dispersed phase solvent by diluting the emulsion by continuous addition of continuous phase solvent. Evaporation of the dispersed phase solvent from the continuous phase/air interface is allowed to occur simultaneously with the extraction process, to maintain a driving force for extraction of the dispersed phase solvent into the continuous phase from the dispersed phase droplets. Uncontrolled evaporation of solvent from the open extraction vessel into the atmosphere is not practical or safe for production of greater than laboratory scale quantities, especially in a continuous process. In a closed vessel, the evaporation would rapidly cease as the solvent in solution equilibrated with the solvent vapor in the head space above the liquid surface. The Li et al. model also demonstrates that large extraction volumes are needed to operate the extraction process in relatively short time periods, as described in U.S. Pat. No. 4,389,330 to Tice et al., although the model predicts that skin formation and the glassy boundary can be achieved using total extraction solvent volumes that are less than the amount needed to dissolve all of the dispersed phase solvent.
It is therefore an object of this invention to provide methods for making microparticles efficiently encapsulating active agent.
It is another object of this invention to provide methods for making microparticles using a process that uses evaporation, controlled extraction, or a combination thereof to minimize the amount of extraction medium required in the process.
It is a further object of this invention to provide alternative methods of emulsion formation and solvent removal for use in processes of making microparticles.
It is another object of this invention to provide emulsion-based methods for making microparticles in an efficient batch or continuous process.